Cryptococcus neoformans is a fungal pathogen that infects the lungs and can disseminate to the central nervous system to cause life-threatening meningitis. There is a lack of effective anti-cryptococcal therapeutics, including vaccines and drugs, due in part to an incomplete understanding of cryptococcal disease mechanisms and host-pathogen interactions. My group has recently shown that a subunit of the SCFFbp1 E3 ubiquitin ligase, Fbp1, is essential for fungal pathogenicity in a manner independent of known virulence factors, suggesting that ubiquitin-mediated protein turnover is involved in a novel virulence control mechanism. We found that Cryptococcus mutants lacking Fbp1 had significantly reduced fungal survival in the lung in murine models and were strongly impaired for survival within macrophages. Interestingly, the fbp1? mutant also elicited a superior host protective immune response compared to the wild type strain, suggesting that Fbp1 may function as or may regulate an immune suppressor. Together, these discoveries led to our central hypothesis that SCFFbp1 E3 ligase regulates fungal pathogenesis by controlling the ubiquitination and degradation of specific proteins - a process required for immune suppression and disease establishment in the lung. In support of this hypothesis, we have already identified one Fbp1 substrate, sphingolipid metabolism enzyme Isc1, and confirmed its role in cryptococcal virulence. Our long-term goal is to understand the molecular mechanism of SCFFbp1 E3 ligase-mediated Cryptococcus-host interaction during lung infection. We propose two Specific Aims. In Aim 1 we will determine how Fbp1 modulates host immunity during lung infection by: (a) deciphering the distinct inflammatory responses triggered in macrophages in response to fbp1? as compared to the parental strain, and (b) evaluating the potential of employing fbp1? as a vaccine strain to promote protection against cryptococcis. In Aim 2 we will characterize the molecular basis of SCFFbp1 E3 ligase-mediated regulation of fungal pathogenesis by (a) characterizing the role of Isc1-dependent sphingolipid biosynthesis in Cryptococcus virulence, and (b) identifying additional Fbp1 substrates that are required for pathogenesis. This project is innovative because it investigates a novel virulence determining mechanism by focusing on a heretofore unknown link between cryptococcal protein turnover and virulence. The proposed research is significant because it will enhance our understanding of E3 ligase-mediated fungal virulence, facilitating efforts to design new therapeutic agents that interact with Fbp1 and its downstream effectors, and may provide a potential vaccine strategy.